They say the attack happened between 5.30am and 6am on Sunday, June 5 outside Premier Cars on Silver Street, Halifax.
The victim appeared to suffer facial injuries but left the scene before police arrived.
He is described as a white man, approximately 30 to 40 years old, of slim build and wearing a thick parka-style coat.
He was last seen walking across the pedestrian crossing near to Bull Green, heading away from the town centre.
A 17-year-old male has been arrested in connection with the offence and released on bail.
The victim of this assault or anyone who believes they know his identity are asked to contact the Calderdale District Investigation Team by using 101 Live Chat on the West Yorkshire Police website or calling 101.